The present invention relates generally to welding. It relates more particularly to an improved method and apparatus for welding plastics and like materials utilizing electromagnetic radiation.
Welding is commonly used to join plastic or resinous parts, such as automobile thermoplastic parts, at a welding zone. Lasers have been used to provide the heat necessary to perform the welding operation (for example, see U.S. Pat. No. 4,636,609 which is expressly incorporated herein by reference). Lasers provide a focused beam of electromagnetic radiation at a specified frequency (i.e., coherent monochromatic radiation). However, lasers tend to be more expensive relative to other heating sources.
Less expensive heating sources, such as infrared heating lamps, are also used to provide infrared radiation for heating the welding zone. One such process is Through-Transmission Infrared Welding (TTIR). The TTIR technique utilizes infrared radiation which passes through at least one plastic part (sometimes called the "transmission piece") in order to heat the welding zone and to provide sufficient heat to join at least two parts.
Infrared heating lamps emit noncoherent radiation which has a broader frequency range (i.e., polychromatic) than laser sources. Some of the frequencies within that broader range produce undesirable results. These undesirable results include heating portions of the parts which are outside of the desired or targeted welding zone. This can result in undesirable effects, including deformation and marking of the overall part.
Solid filters have been used to absorb these undesirable frequencies before the radiation reaches the parts. However, solid filters suffer from several disadvantages, including uncontrolled build-up of heat, as well as having to be continually replaced due to the damage or degradation caused by the radiation or heat by-product. The degradation of the solid filter over a period of time inhibits the performance of the solid filter to absorb the undesirable wavelengths. It is also difficult to adequately cool solid filters in many applications.
A primary object of the present invention resides in the provision of an improved welding apparatus and method utilizing a filter of unique design which obviates the disadvantages of the aforementioned prior filters and is less costly.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawing in which: